High Purity Transparent Perfluoroelastomer Parts and a Process to Produce the Same

ABSTRACT

Crosslinkable perfluoroelastomer compositions having low metal content and low compression set when crosslinked, and processes for producing the same, are provided. Compositions comprising terpolymers of TFE, PAVE, and CNVE having a metal content of less than 3000 ppb may be formed into high purity transparent perfluoroelastomer parts.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/136,853,filed May 25, 2005.

BACKGROUND OF THE INVENTION

Perfluoroelastomers have achieved outstanding commercial success and areused in a wide variety of applications in which severe environments areencountered, in particular those end uses where exposure to hightemperatures and aggressive chemicals occurs. For example, thesepolymers are often used in seals for aircraft engines, in semiconductormanufacturing equipment, in oil-well drilling devices, and in sealingelements for industrial equipment used at high temperatures.

The outstanding properties of perfluoroelastomers are largelyattributable to the stability and inertness of the copolymerizedperfluorinated monomer units that make up the major portion of thepolymer backbones in these compositions. Such monomers includetetrafluoroethylene (TFE) and perfluoro(alkyl vinyl)ethers (PAVE). Inorder to develop elastomeric properties fully, perfluoroelastomers aretypically crosslinked, i.e. vulcanized. To this end, a small percentageof cure site monomer is copolymerized with the perfluorinated monomerunits. Cure site monomers containing at least one nitrile group, forexample perfluoro-8-cyano-5-methyl-3,6-dioxa-1-octene are especiallypreferred. Such compositions are described, for example, in U.S. Pat.Nos. 4,281,092; 4,394,489; 5,789,489; and 5,789,509.

The polymerization processes of perfluoroelastomers are most typicallydone in the presence of a perfluoro carboxylic acid salt or fluorinatedsulfonic acid salt. If the salt contains a metal ion, it contaminatesthe formed polymer. If the salt is a non-metal, usually the resulting pHof the polymerization media is acidic leading to corrosion ofpolymerization vessel or downstream lines and vessels, and subsequentcontamination of the resulting polymer. Further, coagulation of theemulsion or dispersion is usually accomplished by use of magnesium,barium, or other metallic salts resulting in two distinct problems.First, they add metallic contamination to the elastomeric crumb andsecond, the metallic salts of the perfluoro carboxylic acids become muchmore difficult to remove from the crumb.

The prior art further teaches compounding the perfluoroelastomer, forexample, on a roll mill, Banbury mixer, extruder, or the like. In thisstep, crosslinking catalysts or other additives may be mixed with theperfluoroelastomer crumb in the melt to facilitate sufficientcrosslinking as may be required by the application. For example, onegoal may be to attain sufficient crosslinking to achieve good hightemperature compression set resistance. Compounding may actually resultin the addition of metallic and/or other contaminants by the directaddition via additives; additionally high temperature melt compoundingoften results in metal contamination by corrosion of the compoundingequipment and exposure to environmental contamination. If organiccrosslinking agents are used, the resulting articles are usually browndue to thermal decomposition of the agents.

Perfluoroelastomer articles such as seals, O-rings, and valve packingsare often highly filled with carbon black or metallic fillers forreinforcement rendering them opaque and providing an additional sourceof contamination. When exposed to plasmas in end uses such assemiconductor manufacturing, the polymeric component of these articlesis etched away, leaving the fillers as undesirable particlecontaminants. Furthermore, as the polymer decomposes any fillers such asmetals, metal oxides or metal salts originally contained in articles maybe released.

Recent patents of Saito et al. and Coughlin and Wang (U.S. Pat. No.5,565,512, and WO 02/48200) have discussed the value of producing clearand pure perfluoroelastomer parts with low metal ion contamination.Market forces that are driving the move to clear, cleanperfluoroelastomer parts include both the semi conductor industry andthe pharmaceutical industry which desires extremely low concentrationsof metals. In addition, the pharmaceutical and biotechnology industriesdesire overall purity and elimination of certain perfluoro carboxylicacids which accumulate in the body is highly desirable. For example,some companies manufacturing fluoropolymer resins or parts haveestablished limits of perfluoro octanoic acid (PFOA), the acid form ofammonium perfluoro octanoate (APFO) which is a common surfactant used influoromonomer emulsion polymerization.

However, the need for crosslinkable perfluoroelastomers and crosslinkedparts that have a low metallic ion contamination and a low perfluorocarboxylic concentration has not been met with the usual processes offorming these. Therefore, one embodiment of the present invention is amethod for producing perfluoroelastomer compositions having low metallicion contamination and low perfluoro carboxylic concentration.

SUMMARY OF THE INVENTION

This invention relates to crosslinkable perfluoroelastomers and curedperfluoroelastomer articles having low metallic ion concentration and alow concentration of residual fluorosurfactant, and inventive processesfor making the same. In the absence of additives, transparent articleshaving high purity are produced by the methods of the present invention.

In one embodiment, methods of the present invention minimizecontamination in part by minimizing corrosion that results fromconventional polymerization processes performed in the presence ofperfluorocarboxylic acid salt by using a non-metallic buffer and/orcorrosion resistant vessel and/or lines. Corrosion resistant materialsuseful in the present invention include high Ni alloys, for example,Inconel® or Hastelloy® alloys. Processes of present invention may alsosolve the problem of contamination encountered by coagulation of theemulsion or dispersion using metallic salts. For example, by usingnitric acid (HNO₃) or ammonium salts like (NH₄)₂CO₃ and NH₄NO₃ ascoagulants, metallic contamination can be minimized or eliminated. Knownmethods for curing elastomeric resin may result in contamination byusing compounding steps that add metallic and/or other contaminants, orby corrosion of the compounding equipment, or exposure to environmentalcontamination. It has been unexpectedly discovered thatperfluoroelastomeric uncrosslinked gum, having a low concentration ofperfluoro carboxylic acids or salt containing perfluoro cyano vinylether crosslink sites, such as 8-CNVE, can be cured in the mold at about250° C., or greater than 250° C., without a compounding step and withoutthe addition of any other chemicals.

Combining these inventive steps results in the production of crosslinkedperfluoro elastomer parts having metallic ion contamination more than afactor of 100 or a factor of 1000 lower than currently known. Forexample, in one embodiment of the present invention crosslinkedperfluoroelastomeric parts are produced having less than about 3 partsper million (ppm) or more preferably, less than about 0.5 ppm metallicion. The concentration of perfluoro carboxylic acid also may be lessthan about 2 ppm, or less than about 1 ppm. Advantageously, crosslinkedparts of the present invention may have compression set values measuringless than or equal to about 35% at about 200° C. Preferred crosslinkedparts are transparent and colorless.

DESCRIPTION OF THE FIGURES

FIG. 1. Curing kinetics of samples according to Example 2 obtained at250° C.

FIG. 2. Curing kinetics of samples according to Example 4 obtained at250° C.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, the present invention is directed to a compositioncomprising a crosslinkable perfluoroelastomer terpolymer consistingessentially of TFE, PAVE and a cure site monomer having at least onenitrile-containing group; thus, the crosslinkable composition forms acrosslinked terpolymer without additional materials such as crosslinkingagents and the like. The present invention is further directed tomethods of making the crosslinkable terpolymer, methods of crosslinkingthe terpolymer in the absence of a crosslinking agent, and articles madetherefrom.

In one embodiment, perfluoroelastomers of the present invention maycomprise crosslinkable terpolymers polymerized from monomer unitsconsisting essentially of TFE, PAVE, and perfluorocyano vinyl ether. Inone embodiment the PAVE monomer is perfluoromethylvinyl ether (PMVE),however, other suitable perfluorinated vinyl ethers may also be selectedfrom monomers, or mixtures of monomers, of the formula

CF₂═CFO(Rf′O)_(n)(Rf′O)_(m)Rf  (I)

where Rf′ and Rf″ are different linear or branched perfluoroalkylenegroups of 2-6 carbon atoms, m and n are independently 0-10, and Rf is aperfluoroalkyl group of 1-6 carbon atoms.

Another class of perfluorovinyl ethers for use in the present inventionincludes compositions of the formula

CF₂═CFO(CF₂CFXO)_(n)Rf  (II)

where X is F or CF₃, n is 0-5, and Rf is a perfluoroalkyl group of 1-6carbon atoms.

A further class of perfluorovinyl ethers includes those ethers wherein nis 0 or 1 and Rf contains 1-3 carbon atoms. Examples of suchperfluorinated ethers include PMVE, perfluoroethyl vinyl ether (PEVE)and perfluoropropyl vinyl ether (PPVE). Other useful monomers includecompounds of the formula

CF₂═CFO[(CF₂)_(m)CF₂CFZO]_(n)Rf  (III)

where Rf is a perfluoroalkyl group having 1-6 carbon atoms, m=0 or 1,n=0-5, and Z=F or CF₃. Preferred members of this class are those inwhich Rf is C₃F₇, m=0, and n=1.

Additional perfluorovinyl ether monomers for use in the presentinvention may include compounds of the formula

CF₂═CFO[(CF₂CFCF₃O)_(n)(CF₂CF₂CF₂O)_(m)(CF₂)_(p)]C_(x)F_(2x+1)  (IV)

where m and n independently=1-10, p=0-3, and x=1-5. Preferred members ofthis class include compounds where n=0-1, m=0-1, and x=1.

Another example of a useful perfluorovinyl ether includes

CF₂═CFOCF₂CF(CF₃)O(CF₂O)_(m)C_(n)F_(2n+1)  (V)

where n=1-5, m=1-3, and where, preferably, n=1.

Crosslinkable terpolymers of the present invention have cure sitemonomers containing at least one nitrile group. In one embodiment, themonomers include fluorinated olefins containing at least one nitrilegroup, and in another embodiment, the monomers comprisenitrile-containing fluorinated vinyl ethers, including those having thefollowing formulae.

CF₂═CF—O(CF₂)_(n)—CN  (VI)

where n=2-12, preferably 2-6;

CF₂═CF—O[CF₂—CFCF₃—O]_(n)—CF₂—CF(CF₃)—CN  (VII)

where n=0-4, preferably 0-2;

CF₂═C F—[OCF₂CF(CF₃)]_(x)—O—(CF₂)_(n)—CN  (VIII)

where x=1-2, and n=1-4; and

CF₂═CF—O—(CF₂)_(n)—O—CF(CF₃)CN  (IX)

where n=2-4. Particularly preferred cure site monomers areperfluorinated polyethers having a nitrile group and a trifluorovinylether group, including perfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene),

CF₂═CFOCF₂CF(CF₃)OCF₂CF₂CN  (X).

Preferred perfluoroelastomer compositions of the present invention arecomprised of a crosslinkable terpolymer consisting essentially of unitsof TFE, PAVE and cure site units having at least one nitrile-containinggroup, where in one embodiment PAVE is PMVE and further, wherein8-perfluorocyano vinyl ether (8-CNVE) is the nitrile-containing curesite monomer. The crosslinkable terpolymer may be polymerized from theabove monomers by known methods including those described in WO02/060968 to Coggio et al. which is hereby incorporated by referenceherein, and further, methods as described in detail in the examplespresented below. In one embodiment, crosslinkable perfluoroelastomerterpolymers consist essentially of approximately from 38 to 81.7 molepercent TFE, 18 to 58 mole percent PAVE, and 0.3 to 4 mole percent of anitrile-containing cure site monomer. Other crosslinkable terpolymers ofthe present invention consist essentially of about 47 to 80 mole percentTFE, 19 to 50 mole percent PAVE, and 1 to 3 mole percentnitrile-containing cure site monomer.

After polymerization to form crosslinkable terpolymers of the presentinvention, the gum may be further processed with a finishing step asdescribed in Example 1 below which may facilitate the elimination ofsome contaminants.

In one embodiment, the highly pure crosslinkable terpolymers have lowmetal ion content (or metal contamination), as well as lowfluorosurfactant concentration. The metal content of the crosslinkableterpolymer is less than 200 ppm, and preferably less than 3000 parts perbillion (ppb), also preferred less than about 2000 ppb, furtherpreferred less than about 1000 ppb, more preferably less than about 500,and most preferably less than about 200 ppb when measured according tothe methods described herein for determining metal content. The metalcontent of preferred crosslinked terpolymer is also less than 200 ppm,preferably less than 3000 ppb, more preferably less than about 2000 ppb,further preferred less than about 1000 ppb or less than about 500 ppbwhen measured according to the methods described herein for determiningmetal content. In one embodiment, the fluorosurfactant concentration ispreferably less than 2 ppm for one or both of the uncrosslinked andcrosslinked terpolymer, when measured according to the methods describedherein. Preferably, the concentration of perfluoro carboxylic acid maybe less than about 2 ppm, and less than 1 ppm. Uncrosslinked andcrosslinked terpolymers may have a fluoro sulfonic acid concentration ofless than about 2 ppm, or less than 1 ppm. APFO concentrations ofuncrosslinked and crosslinked compositions may be less than 2 ppm or ina further embodiment less than 1 ppm.

The present invention is further directed to a process for making highlypure crosslinked perfluoroelastomeric articles. One embodiment of thepresent invention comprises a method comprising heating a compositioncomprising a crosslinkable terpolymer consisting essentially of TFE,PAVE, and nitrile-containing cure-site monomer units, to form highlypure crosslinked composition to which no crosslinking agents have beenadded. One method comprises:

1) forming a composition comprising a crosslinkable perfluoroelastomericterpolymer of the present invention consisting essentially of a) TFE, b)PAVE, and c) nitrile-containing cure site monomer;

2) shaping the crosslinkable perfluoroelastomeric terpolymercomposition;

3) heating said shaped perfluoroelastomeric terpolymer composition, and

4) crosslinking the perfluoroelastomer terpolymer by heating, whereinthe process is performed without adding or, in the absence of, acrosslinking agent.

The method of the present invention may include shaping by molding orother fabrication techniques by means that do not introduce significantmetallic contamination.

In one embodiment, the method comprises heating and crosslinking theterpolymer having units with nitrile-containing cure sites in theabsence of, or without the addition of, one or more crosslinking agents,until sufficient crosslinking is achieved. Crosslinking agents includingcoagents, catalysts, and the like (such as peroxides, isocyanurates,ammonia-generating compounds, and bisamidoxime) that are typically usedfor curing crosslinkable polymers, impart contaminants, and are notnecessary for crosslinking terpolymers using the novel methods of thepresent invention. The exclusion of these crosslinking agents from themethod of the present invention results in crosslinked compositionshaving higher purity than achieved by currently known methods. Preferredcrosslinked perfluoroelastomers are translucent or transparent afterheating.

In one embodiment, the method comprises heating and crosslinking shapedperfluoroelastomer to greater than or equal to about 250° C. in theabsence, or without the addition of crosslinking agents or additives,until sufficient crosslinking is achieved; in a further embodiment, themethod comprises heating to greater than or equal to about 300° C., inthe absence of, or without the addition of, crosslinking agents. Heatingand crosslinking are maintained at temperatures and for times sufficientto cure the terpolymer to a desired level. In a further embodiment, theheating and crosslinking are continued for times and temperaturesnecessary to obtain a specific compression set. For example, the methodcomprises heating and crosslinking until a crosslinked terpolymer orshaped article is formed having a compression set of less than or equalto about 50% when tested at about 200° C. according to the methoddescribed herein. In other embodiments the method comprises heating andcrosslinking until a crosslinked terpolymer or shaped article has acompression set of less than or equal to about 40%, less than or equalto about 35%, less than or equal to about 30%, or less than or equal toabout 10% when tested at about 200° C. according to the method usedherein, and described below. A crosslinkable terpolymer composition maybe heated, for example, for about 30 minutes or greater, or for about 60minutes or greater, at a temperature of greater than about 250° C. orgreater than or equal to about 300° C., to achieve these properties.Preferred crosslinked compositions of the present invention have acompression set less than or equal to about 40%, and more preferablyless than or equal to about 35%, when tested at about 200° C. accordingto the method described herein.

For use in evaluating the crosslinked compositions, compression set ismeasured according to ASTM D 395-01 Method B, at approximately 25%deflection, for about 70 hours in air. Articles are taken off from thetesting device and reheated to the testing temperature for one (1) hourand measured.

Articles made from the perfluoroelastomer terpolymer of the presentinvention are useful in applications requiring higher purity than can beobtained by currently known methods. A few uses of articles formed fromcompositions of the present invention include gaskets such as o-rings,tubes, diaphragms, seals and the like. Crosslinkable terpolymers of thepresent invention may be shaped and cured directly into usable articles.

Test Methods

APFO Analysis

The methanolic HCl derivitization method is used to change the APFO formfrom the salt or carboxylic acid into its methylester derivative. Thisform is easily analyzed via Gas Chromatography (GC).

The APFO in about 1 g polymer is extracted and derivatized into 10 mlMethanolic HCl (Part # 33050-U, Supelco) over two hours at 55° C. Thederivative mixture is then combined with 20 ml of half saturatedNaCl/aqueous solution (98+%, Sigma Aldrich) and 10 ml n-Hexane (99+%,Sigma Aldrich). The derivative is extracted into the Hexane layer, whichis then removed for GC analysis.

The GC analysis is performed splitless using a non-polar column and anElectron Capture Detector (Examples 2, 3 and 4).

EXAMPLES Example 1

An aqueous emulsion containing 10 g 8-CNVE[CF₂═CF—O—(CF₂)₃—O—CF(CF₃)—CN], 135 g deionized (DI) water and 5 g 20 wt% ammonium perfluorooctanoate (APFO) aqueous solution was prepared byusing an Omini Mixer Homogenizer (Omini International Co.) for 5minutes. This solution is designated as “stock solution A”.

Approximately 1500 g DI water, 300 g 20 wt % APFO aqueous solution and16 g 8-CNVE were charged into an oxygen-free 4-liter reactor. Then, 190g TFE and 300 g PMVE were added into the reactor. The reactor was thenheated to 70° C. under 2285 KPa and the polymerization reaction wasinitiated by feeding 202 g ammonium persulfate (APS) aqueous solution (2g APS dissolved in 200 g DI water) within 2 minutes. As the reactionpressure decreased to 1800 KPa, 105 g stock solution A with 120 g DIwater and 20 g TFE were charged into the reactor within 3 minutes. Then,150.5 g APS solution (0.5 g APS dissolved in 150 DI water) was fed intothe reactor within 1 minute. As the reaction pressure decreased to 1600KPa, 45 g stock solution A with 150 g DI water and 20 g TFE were chargedinto the reactor within 1 minute. Then, 150.5 g APS solution (0.5 g APSdissolved in 150 g DI water) was added into the reactor within 1 minute.The polymerization reaction was stopped after 221 minutes from theinitiation of the reaction under 518 KPa. The reactor was cooled and theresidual gas was purged. The emulsion latex containing 16.9 wt % solidswas obtained.

Finishing Process 1:

Approximately 10 ml nitric acid (minimum 65%, semiconductor grade,Riedel-deHaen) was introduced into 200 ml of the emulsion latex(prepared substantially according to Example 1) in a polypropylene (PP)beaker with stirring at room temperature. The liquids were decanted andthen the precipitated solids were immersed in 200 ml methanol(semiconductor grade, Riedel-deHaen) at room temperature. After 24hours, the methanol was decanted and the polymer was washed with 200 mlmethanol (semiconductor grade, Riedel-deHaen). The polymer was dried at120° C. for 12 hours in a convection oven.

Finishing Process 2:

The procedure is the same as the above, but the nitric acid used was anACS reagent grade (70%, Aldrich) and the methanol used was a PRA grade(99.9%, Aldrich).

The 2 dried polymer samples were analyzed by Inductively CoupledPlasma-Mass Spectroscopy (ICP-MS) for 16 metal elements. Table 1 liststhe metal ion levels in the polymers.

Solid-state ¹⁹F NMR was carried out to characterize the composition ofthe polymer. This polymer sample contained 62.4 mol % TFE, 36.6 mol %PMVE and 1.0 mol % 8-CNVE.

Example 2

An aqueous solution containing 10 g 8-CNVE[CF₂═CF—O—(CF₂)₃—O—CF(CF₃)—CN], 136 g DI water and 4 g of 20 wt % APFOaqueous solution was prepared by using an Omini Mixer Homogenizer for 5minutes. This solution is designated as “stock solution B”.

Approximately 1500 g DI water, 300 g 20 wt % APFO aqueous solution and16 g 8-CNVE were charged into an oxygen-free 4-liter reactor. Then, 190g TFE and 320 g PMVE were added into the reactor. The reactor was thenheated to 70° C. under 2347 KPa and the polymerization reaction wasinitiated by feeding 200.5 g APS aqueous solution (0.5 g APS dissolvedin 200 g DI water) within 1 minute. As the reaction pressure decreasedto 1900 KPa, 105 g stock solution B with 120 g DI water and 20 g TFEwere charged into the reactor within 2 minutes. As the reaction pressuredecreased to 1700 KPa, 45 g stock solution B with 150 g DI water and 20g TFE were charged into the reactor within 2 minutes. The polymerizationreaction was stopped after 367 minutes from the initiation of thereaction under 600 KPa. The reactor was cooled and the residual gas waspurged. The emulsion latex containing 18.2 wt % solids was obtained.

Approximately 400 ml of the emulsion latex was coagulated at roomtemperature with 20 ml nitric acid (70%, ACS reagent, Aldrich) in a PPbeaker. The liquids were decanted and then the precipitated material wasimmersed in 400 ml methanol (99.9%, PRA grade, Aldrich) for 24 hours atroom temperature. Then, the methanol was decanted and the material waswashed with 400 ml methanol (99.9%, PRA grade, Aldrich). The methanolwas decanted and the washed material was dried at 70° C. for 48 hours ina convection oven.

The APFO residual detected from the polymer was 0.3 ppm. Solid-state ¹⁹FNMR showed it had 61.7 mol % TFE, 37.3 mol % PMVE and 1.0 mol % 8-CNVE.

An ARES rheometer (Rheometrics) was used to monitor the curing process.Disks having an 8 mm diameter and about a 0.8 mm thickness were moldedfrom the polymer at 100° C. for 2 minutes. A disk was placed between two8 mm diameter parallel plates at 60° C. for 100 seconds and then heatedto a setting curing temperature from a starting temperature of 60° C. ata heating rate of 80° C./min. Curing was carried out at a frequency of10 rad/second, a strain of 0.1% and a setting temperature in air. Torqueand Tan δ=G″/G′ were monitored with time, where G′ is the storage shearmodulus and G″ the loss shear modulus. Its curing curve is shown in FIG.1.

The crumb polymer was molded into AS-568A K214 (Aerospace StandardO-ring size) O-rings at 300° C. and 1727 psi for 1 hour and then werepostcured in air at 300° C. for 24 hours. The O-rings made weretransparent.

Compression set was measured on O-rings largely based on ASTM D 395-01Method B. However, the ASTM method does not have a quantitative time ortemperature scale as to how soon or at what temperature the testedspecimens should be taken off from the testing device. Differentcompression set values can be obtained when tested specimens are takenoff from the testing device at different temperatures. To avoid thisissue, tested specimens taken off from the testing device were reheatedto the testing temperature for 1 hour, and then measured according toASTM D 395-01, i.e., cooling for 30 minutes, etc. The compression setvalue is given in Table 2.

Example 3

An aqueous solution containing 10 g 8-CNVE[CF₂═CF—O—(CF₂)₃—O—CF(CF₃)—CN], 480 g DI water and 10 g 20 wt % APFOaqueous solution was prepared by using an Omini Mixer Homogenizer for 5minutes. This solution is designated as “stock solution C”.

Approximately 1500 g DI water, 300 g 20 wt % APFO aqueous solution and16 g 8-CNVE were charged into an oxygen-free 4-liter reactor. Then, 260g TFE and 300 g PMVE were added into the reactor. The reactor was thenheated to 70° C. under 2584 KPa and the polymerization reaction wasinitiated by feeding 200.2 g APS aqueous solution (0.2 g APS dissolvedin 200 g DI water) within 1 minute. Then, stock solution C was fed intothe reactor as follows:

Time after reaction initiation Stock solution C added (in minutes) (ingrams) 2 60 16 60 28 60 40 60 51 50 61 60 72 60 83 80 98 10

As the reaction pressure decreased to 2120 KPa, 20 g TFE was chargedinto the reactor within 1 minute. Another 20 g TFE was added into thereactor within 1 minute as the reaction pressure decreased to 1920 KPa.The polymerization reaction was stopped after 219 minutes from theinitiation of the reaction under 1200 KPa. The reactor was cooled andthe residual gas was purged. The emulsion latex containing 15.9 wt %solids was obtained.

The coagulation process was substantially the same as the firstfinishing process as shown in Example 1. The polymer was dried at 70° C.for 48 hours in a convection oven.

The dried polymer sample was analyzed by ICP-MS for 16 metal elements.Table 1 lists the metal ion levels in the polymer.

The APFO residual detected from the polymer was 1.2 ppm. This polymerhad 74.9 mol % TFE, 24.2 mol % PMVE and 0.9 mol % 8-CNVE, as determinedby solid-state ¹⁹F NMR.

The crumb polymer was molded into AS-568A K214 O-rings, heating at 300°C. and 1658 psi for 5 minutes, and then was postcured in air at 250° C.for 24 hours. The O-rings made were transparent. The compression setvalue is given in Table 2. The crumb polymer was also molded and curedinto 1 mm thick films between Kapton® films under the same molding,heating and postcuring condition. The purity of the crosslinked film isshown in Table 1.

Example 4

Approximately 1800 g DI water and 180 g 20 wt % APFO aqueous solutionwere charged into an oxygen-free 4-liter reactor. Then, 3.6 g 8-CNVE[CF₂═CF—O—(CF₂)₅—CN], 76 g PMVE and 62.8 g TFE were added into thereactor.

The reactor was heated to 60° C., and then the mixture of TFE with PMVE(55/45, wt/wt) was charged into the reactor until the pressure increasedto 920 KPa. Then 200 ml aqueous solution containing 6 g APS and 4 g 25wt % ammonium sulfite was added into the reactor to initiate thepolymerization reaction.

Once the initiation reaction started, 8-CNVE was continuously chargedinto the reactor at a rate of 0.143 g/min, and the mixture of TFE withPMVE (55/45 wt/wt) was also continuously supplied to the reactor tomaintain the reaction pressure at 930-950 KPa.

After 440 minutes from the start of the reaction initiation, the supplyof 8-CNVE and the mixture of TFE with PMVE was then stopped. The reactorwas kept in that state for another hour. Then reactor was cooled and theresidual gas was purged. The emulsion latex containing 27.5 wt % solidswas obtained.

The coagulation process is the same as the first finishing process asshown in Example 1. The polymer was dried at 70° C. for 48 hours in aconvection oven.

The dried polymer sample was analyzed by ICP-MS for 16 metal elements.Table 1 lists the metal ion levels in the polymer.

The APFO residual detected from the polymer was 0.8 ppm. Solid-state ¹⁹FNMR was carried out to characterize the composition of the polymer. Thispolymer sample contained 69.6 mol % TFE, 29.2 mol % PMVE and 1.2 mol %8-CNVE.

An ARES rheometer (Rheometrics) was used to monitor the curing process.Disks having an 8 mm diameter and about a 0.8 mm thickness were moldedfrom the polymer at 100° C. for 2 minutes. A disk was placed between two8 mm diameter parallel plates. Curing was carried out at a frequency of10 rad/second, a strain of 0.5% and heating at about 250° C. in air.Torque and Tan δ=G″/G′ were monitored with time. Its curing curve isshown in FIG. 2.

The crumb polymer was molded into AS-568A K214 O-rings heating at 250°C. and 1727 psi for 30 min and then was postcured in air at 90° C. for 4hours, 204° C. for 24 hours and 288° C. for 24 hours. The O-rings madewere transparent. The compression set value is given in Table 2. Thecrumb polymer was also molded into 1 mm think films between Kapton®films under the same molding and postcuring condition. The purity of thecrosslinked film is shown in Table 1.

TABLE 1 Metal ions detected in the crosslinkable polymers and thecrosslinked parts. Ex. 1 ⁽¹⁾ Ex. 1 ⁽²⁾ Ex. 3 ⁽³⁾ Ex. 3 ⁽⁴⁾ Ex. 4 ⁽⁵⁾ Ex.4 ⁽⁶⁾ Metal Level Detected Level Detected Level Detected Level DetectedLevel Detected Level Detected Ions (ppb) (ppb) (ppb) (ppb) (ppb) (ppb)Al 1 <1 <1 12 1 8 Ba <1 <1 1 1 <1 <1 Ca 37 15 50 100 20 70 Cr 6 <5 <5 <5<5 13 Cu <5 <5 <5 <5 <5 <5 Fe 17 10 <10 <10 <10 30 Pb <1 <1 <1 <1 <1 <1Li 1 3 <1 <1 <1 <1 Mg 1 1 18 29 12 23 Mn 1 1 2 3 <1 2 Ni 36 37 16 14 2733 K 11 <10 <10 <10 <10 10 Na 70 8 22 26 9 200 Sr <1 <1 <1 <1 <1 <1 Ti<10 <10 <10 <10 <10 <10 Zn <10 <10 <10 <10 <10 <10 ⁽¹⁾ The polymerobtained by finishing process 2. ⁽²⁾ The polymer obtained by finishingprocess 1. ⁽³⁾ The crumb polymer. ⁽⁴⁾ The crosslinked film. ⁽⁵⁾ Thecrumb polymer. ⁽⁶⁾ The crosslinked film.

TABLE 2 Compression set values. Compression set, %* Example 2 65 Example3 35 Example 4 7 *25% deflection, 70 hours in air at about 204° C.

1. A composition comprising a crosslinkable perfluoroelastomercomposition consisting essentially of a terpolymer oftetrafluoroethylene (TFE), perfluoroalkyl vinyl ether (PAVE) andperfluoro cyano vinyl ether (CNVE), wherein the crosslinkableperfluoroelastomer composition has less than about 3000 ppb metalcontent, and further wherein the composition, when crosslinked has acompression set less than or equal to about 50% when tested at about200° C.
 2. The composition of claim 1, wherein the crosslinkableperfluoroelastomer has less than about 1000 ppb metal content.
 3. Thecomposition of claim 1, wherein the crosslinkable perfluoroelastomer hasless than about 500 ppb metal content.
 4. The composition of claim 1,wherein the crosslinkable perfluoroelastomer has less than about 200 ppbmetal content.
 5. The composition of claim 1, wherein PAVE isperfluoromethyl vinyl ether (PMVE).
 6. The composition of claim 1,wherein PAVE is perfluoroethyl vinyl ether (PEVE).
 7. The composition ofclaim 1, wherein PAVE is perfluoropropyl vinyl ether (PPVE).
 8. Thecomposition of claim 1 wherein the perfluoro cyano vinyl ether is8-CNVE.
 9. The composition of claim 1, wherein the crosslinkableperfluoroelastomer comprises 0.3-3 mol % CNVE.
 10. An article comprisingthe crosslinked perfluoroelastomer composition of claim
 1. 11. Thearticle of claim 10, wherein the article is a gasket.
 12. The article ofclaim 10, wherein the article is a tube.
 13. The article of claim 10,wherein the article is a diaphragm.
 14. The article of claim 10, whereinthe article is a seal.
 15. The composition of claim 1, wherein thecrosslinkable polymer has a fluorosurfactant concentration of less than2 ppm.
 16. The composition of claim 1, wherein the crosslinkable polymerhas a fluorosurfactant concentration of less than 1 ppm.
 17. Thecomposition of claim 1, wherein the crosslinkable polymer has acarboxylic acid surfactant concentration of less than 2 ppm.
 18. Thecomposition of claim 1, wherein the crosslinkable polymer has acarboxylic acid surfactant concentration of less than 1 ppm.
 19. Thecomposition of claim 1, wherein the crosslinkable polymer has a fluorosulfonic surfactant concentration of less than 2 ppm.
 20. Thecomposition of claim 1, wherein the crosslinkable polymer has a fluorosulfonic acid surfactant concentration of less than 1 ppm.
 21. Thecomposition of claim 1, wherein the crosslinkable polymer has afluorosurfactant concentration of less than 2 ppm when crosslinked. 22.The composition of claim 1, wherein the crosslinkable polymer has afluorosurfactant concentration of less than 1 ppm when crosslinked. 23.The composition of claim 1, wherein the crosslinkable polymer has acarboxylic acid surfactant concentration of less than 2 ppm whencrosslinked.
 24. The composition of claim 1, wherein the crosslinkablepolymer has a carboxylic acid surfactant concentration of less than 1ppm when crosslinked.
 25. The composition of claim 1, wherein thecrosslinkable polymer has a fluoro sulfonic surfactant concentration ofless than 2 ppm when crosslinked.
 26. The composition of claim 1,wherein the crosslinkable polymer has a fluoro sulfonic acid surfactantconcentration of less than 1 ppm when crosslinked.
 27. The compositionof claim 1, wherein the crosslinkable perfluoroelastomer has less thanabout 2000 ppb metal content when crosslinked.
 28. The composition ofclaim 1, wherein the crosslinkable perfluoroelastomer has less thanabout 1000 ppb metal content when crosslinked.
 29. The composition ofclaim 1, wherein the crosslinkable perfluoroelastomer has less thanabout 500 ppb metal content when crosslinked.
 30. The composition ofclaim 1 having a compression set of less than or equal to about 35% whencrosslinked.